Just Say No to Exposure Compensation in Manual Exposure Mode
Exposure Compensation and manual exposure mode are two great things that don't taste great together. Both give you improved control over exposure, but try to use both at the same time and you'll be in for a surprise.
Many things in the real world aren't medium gray but cameras and exposure meters have no good way of knowing that. Meters are calibrated to render whatever you point them to as eighteen percent reflectance which is the value our eyes see as "medium toned." Meters can tell if one thing is brighter or darker than another and can thus gauge relative brightness, but there's simply no absolute way to gauge how to render brightness. You might think that with all the technology we carry around in our cameras these days science would have solved this problem by now, but if you think about it, it's not really a solvable problem. It's not a bug, it's a feature.
Find something that truly is medium toned gray such as a calibrated gray card, an item often used to establish a frame of reference for exposure. You know your gray card is medium gray because you paid a lot of money to get one that's medium gray. Most are just pieces of colored cardboard, but a good one can cost around twenty bucks. It had better be accurate. But place it in the shadows and it will look dark gray. Put it in bright light and it will surely look brighter gray than it did in the shadows. If you forced both to appear the true medium gray that the card actually is, both would appear to be under the same light. Accurately rendering the tone of the gray card forces you to sometimes misrepresent how bright the ambient lighting is.
And it gets even more complicated. A camera lens crops a scene to include a limited area and exclude everything outside that area. Suppose you have two gray cards positioned near each other, but one is in bright light and the other in shadows. Zoom in so you see only one or the other and you can come up with an exposure to render it medium gray as it should be. But zoom out so your view encompasses both cards and you have a problem. Should the well lit gray card be rendered as medium and the one in the shadows show up as dark gray, or should the gray card in the shadows be rendered as true eighteen percent reflectance making the shadows less murky in the image, but making the gray card in the well-lit area appear brighter than medium? Both gray cards are in fact the same tonality since you paid good money to make sure they are, yet they can't possibly both appear eighteen percent medium gray in the same image given that they are under different lighting.
It's up to you which gray card in the above example, if either, you want to appear truly medium toned. Your camera meter can't decide for you. If you set it on fully automatic exposure, it will come up with some image, but that's only one possible choice that could have been made. If you don't like what it chooses, or just want to keep control yourself rather than delegate everything to the whims of automatic exposure, you have to do something.
The method most people employ since it seems conceptually straightforward is Exposure Compensation. By increasing or decreasing the setting of this single adjustment you can tell your camera to bias the exposure of the resulting image by the value you select. Set the Exposure Compensation to plus one stop and rather than rendering your subject as medium toned your camera meter will bias it to come out one stop over medium. Likewise in reverse if you set it to a value below the zero mid-point. If you generally trust the decisions made by your camera and only feel compelled to influence it occasionally, you'll find Exposure Compensation to be useful. This works similarly in fully automatic mode as well as Aperture Priority and Shutter Priority where you choose one basic exposure variable and allow your camera to determine the other.
An alternate method that gives you complete control of exposure is the use of manual exposure mode. In this mode, you select both aperture and shutter speed yourself, so you control not only exposure but also depth of field, motion blur and other affects influenced by these settings. The camera still meters as normal, but doesn't set either the aperture or shutter speed for you. Instead, all it does is display the brightness reading it measured. If you've chosen a combination of aperture and shutter speed that would render your subject medium, the meter will show zero. If that subject would be a stop over medium, the meter will show plus one, and so on. This seems straightforward in concept, but new users often have a hard time learning to use manual exposure. With both aperture and shutter speed to adjust, and with a change to one forcing an inverse change to the other to keep the exposure the same, getting a handle on manual exposure has been likened to patting your head with one hand while rubbing your stomach with the other. Try it sometime if you don't understand my point. It's sounds easy in concept, but can be harder than you think to master.
Anyway, photographers sometimes attempt to use both these techniques at the same time believing that this would give them even better control than either technique by itself. But if you do, you'll find that nothing seems to work right anymore. Remember, in manual exposure mode, the camera won't change either the aperture and shutter speed on its own. Whatever you set them to, that's where they stay unless you change them. Given this, what's left for Exposure Compensation to do? The camera will still bias its meter reading based on how you set the Exposure Compensation, but this won't affect either aperture or shutter speed.
To understand what does happen, let's first look at how things work when you have your camera set to automatic, Aperture Priority or Shutter Priority mode. If you set Exposure Compensation to minus one stop, you're telling the camera you want it to stop down by one stop to make the image come out darker. Internally, it does this by telling the meter that things are actually one stop brighter than they actually are. The camera then reacts by stopping down the requested one stop, leaving you where you expected to be with an image one stop darker than without.
The same thing would still happen in manual exposure mode, but since you control both the aperture and shutter speed, the camera can't react to the Exposure Compensation change, leaving you with a meter reading one stop above where it started. That's right; you set the Exposure Compensation to minus one, and the meter reading goes up one stop. Set Exposure Compensation to plus one stop and the meter reading will go down by one stop. The meter reacts exactly the opposite way you might intuitively think it would. You want the exposure to be darker and the meter says it's brighter. You want it brighter and the meter reads darker. And the whole time, the exposure won't really change at all since the aperture and shutter speed will remain where you set them. Manual exposure is manual exposure. You control both variables and the camera won't adjust either aperture or shutter speed for you.
In other words, in manual exposure mode, the Exposure Compensation control appears to do the opposite of what you expect, but in reality does nothing at all. The exposure remains unchanged.
Instead, if you want an image to appear one stop darker in manual exposure mode, you should stop down the aperture by one stop or cut the shutter speed in half. When you do, the meter reading will go down by one stop as you expect. Likewise, if you want the exposure to be one stop brighter you can open up the aperture by one stop or double the exposure time and the meter reading will go up one stop to match. All you have to do is change the aperture and/or shutter speed until the meter reads the way you want the exposure rendered. There's neither need nor reason for Exposure Compensation in manual exposure mode.
So the bottom line is, if you shoot in automatic or semi-automatic metering (Aperture Priority or Shutter Priority) use the Exposure Compensation control to influence exposure. But in manual exposure, leave the Exposure Compensation alone and just pay attention to the meter reading as you change the aperture and shutter speed. Never use both manual exposure and Exposure Compensation at the same time.
Updated 9/22/2013 — Reader DO catches me in an oversimplification regarding the commonly reference 18% gray rule. I try to keep my articles accurate even when the underlying details might otherwise become a huge topic. My references to 18% were intended as illustrative only, but yes, the actual value will work out somewhat less. A gray card evenly lit straight on would reflect 18%, but since reflectivity depends in part on the angle of incidence, an average scene does indeed reflect somewhat less than 18%. That's why hand held light meters generally have hemispherical "dome" shaped sensors. You're right then that to compensate, most camera meters are calibrated somewhere between 12 – 18%. Thanks for keeping me honest.